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to aid in testing

This is because if the user specifies that they want
to mount multiple times, something must be wrong. try
unmounting to reset things and then proceed.

This commit adds a Mount abstraction (which is really just
a wrapped context closer). It makes sure to bind the mount
to the fate of the Node (i.e. close it if the node ends).
This fixes #350

@jbenet

ipfs1 docs read:

```
btc λ. ipfs pin
ipfs pin -

Commands:

        add         pin an ipfs object to local storage.
        rm          unpin an ipfs object from local storage.

        Use "pin help <command>" for more information about a command.
```

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

(The file path is omitted, but since only one object can be added at a time, I think this is ok)

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

Also:
- map[cmds.EncodingType]cmds.Marshaller -> MarshalMap

cc @mappum @maybebtc